There is known a wireless network called “wireless multi-hop network (also referred to as “mobile ad-hoc network”)”. In this wireless network, not only that communication terminal pairs directly communicate with each other by wireless signals, but also that a one first communication terminal can communicate with a one second communication terminal located outside the area that the wireless signal of the one first terminal can reach. This wireless communication is achieved using other adjacent communication terminals as data relay nodes of the wireless network.
Thus, the wireless multi-hop network is constituted by a plurality of communication terminals in a self-organizing manner. Further, the communication terminals each have a router function and thereby can perform data relay. As a routing protocol that controls a communication route of the data relay, the followings are known. One is a Reactive type protocol that searches for a communication route so as to determine it at a communication start time. The other is a Proactive type protocol that exchanges information with adjacent communication terminals in a non-communication time to thereby previously determine a communication route. Hereinafter, a Proactive type wireless multi-hop network using the Proactive type protocol will be described.
In a conventional communication route control method for the Proactive type wireless multi-hop network, periodic exchange of a neighbor detection message (referred to as “HELLO” message) is performed and, thereby, the existence of neighboring communication terminals is grasped. Subsequently, a topology message including link information between a plurality of neighboring communication terminals is periodically notified throughout the network. This allows each communication terminal to build a network topology graph to thereby grasp the topology (information indicating connection state between respective communication terminals) of the entire network. Then, the shortest route is calculated based on the topology. In this manner, the communication route control is realized.
Such a method is disclosed in the following Non-Patent Document 1. However, a message needs to be frequently transmitted in this method, resulting in a high message load, so that it is impossible to use this method in a wireless network exhibiting lower bandwidth capability. Further, the frequent message transmission consumes power significantly, which may shorten a power supply life. Further, the message transmission frequency is determined by the movement speed of a communication terminal. Therefore, in a network constituted by communication terminals having different movement speed, the transmission frequency needs to be adjusted to a communication terminal exhibiting the highest movement speed, consuming unnecessary bandwidth. Moreover, in the case where a given communication terminal moves at a higher speed than expected, update of the network topology cannot keep up with demand, inhibiting normal route control.
As a countermeasure against the above problem, the following Non-Patent Document 2 discloses a method in which only a difference in a change in the communication route is transmitted so as to suppress the message load. However, the transmission frequency itself has not been improved also in this method, so that the power consumption of the terminal cannot be suppressed.
Further, in order to suppress the message load, it can be considered that the transmission frequency is lowered. However, when the transmission frequency is lowered, it takes time to update the neighboring terminal information and network topology information, with the result that communication becomes unstable. In order to overcome the above disadvantage, a method called FSR (Fisheye State Routing protocol) that controls the transmission frequency in accordance with the distance (hop count) from the communication terminal is disclosed in the following Non-Patent Document 3. However, correct topology information cannot be obtained in this method and, after all, it is difficult to perform stable communication in a network a movement of a node happens frequently.
Non-Patent Document 1: T. Clause and other one, “Optimized Link State Routing Protocol (OLSR)”, IETF RFC3626, October 2003
Non-Patent Document 2: R. Ogier and other two, “Topology Dissemination Based on Reverse-Path Forwarding (TBRPF)”, IETF RFC3684, February 2004
Non-Patent Document 3: A. Iwata and other four, “Scalable Routing Strategies for Ad-hoc Wireless Networks”, IEEE JSAC, August 1999, Vol. 17, No. 8, p. 1369-1379
Non-Patent Document 4: Zygmunt Haas and other two, “Gossip-based ad hoc routing”, IEEE INFOCOM 2002, 2002